The development of modern endonasal sinus surgery is fundamentally based on the introduction of optical instrumentation. Endoscopes and microscopes are commonly used in such surgery. We tested a new combination of a stereoscopic single-channel endoscope with an operating microscope (ENDO-OPMI) for endonasal sinus surgery. This combination of the endoscope and microscope are connected in a way that results in a self-carried stereo-endoscope. To date, the ENDO-OPMI has been evaluated in cadaver studies and 7 cases of endonasal sinus surgery for chronic sinusitis. The trial was used to evaluate the clinical use of the current system and its possibilities for future developments. The ENDO-OPMI was demonstrated to be principally well-suited for endonasal sinus surgery. However, the current technical shortcomings make further developments necessary. Once these are implemented, the system should be superior to the use of an endoscope or microscope alone in endonasal surgery.